As a driving device for a large number of industrial machines, a general-purpose servo control device is used. Parameters, such as a control gain, are adjusted and set according to uses and characteristics of the machines to realize a desired control performance as much as possible. As motors directly controlled by the servo control device, there are a rotary motor and a linear motor. In this specification, in order to facilitate understanding, the rotary motor is explained. That is, when a motor for driving a controlled object mechanical system is, for example, a rotary motor, the servo control device generates a torque command to the rotary motor by performing position control and speed control by feedback control such that the position and the speed of the controlled object follow target values.
In the servo control device, when a control gain used for the position control and the speed control by the feedback control is increased, follow-up control accuracy to the target values can be improved. However, when the control gain is too large, on the contrary, a phenomenon such as oscillation occurs and the control becomes unstable.
Therefore, the control gain is adjusted and set to be as large as possible within a range in which the control does not become unstable. However, in an actual controlled object, non-linear characteristics are included in driving force transmission mechanisms, such as a gear and a ball screw, and mechanical systems including mechanical resonance, friction, and the like. Therefore, the ease of oscillation is different depending on operation conditions, such as speed and acceleration of a mechanical system driven by a motor.
Therefore, in a conventional servo control device, in general, a control gain having a small value with a margin given to oscillation to prevent control from becoming unstable under various operation conditions is fixedly set. That is, the conventional servo control device performs control by using a constant fixed value control gain and does not change the control gain according to the control content. Therefore, the control performance cannot be further improved. Various proposals have been made to solve such problems (e.g., Patent Literatures 1 and 2).
That is, Patent Literature 1 proposes a technology for, in a servo control device used in position control, switching a control gain to a small value during a stop of a controlled object to thereby increase stability during the stop and relatively increasing the control gain during the operation of the controlled object to realize control higher in speed and accuracy than a control system having a fixed gain.
The servo control device described in Patent Literature 1 performs, based on a deviation between a position command and a position detection value, position PI control applied with a position proportional gain and a position integral gain to generate a speed command and performs, based on a deviation between the speed command and a speed detection value, the speed PI control applied with a speed proportional gain and a speed integral gain to generate a torque command (a current command). The servo control device sets a setting value of a speed level in advance and switches a gain according to a speed detection value to set the position proportional gain relatively high when the speed detection value is larger than the setting value and set the position proportional gain to a relatively low value when the speed detection value is lower than the setting value. That is, the servo control device performs a change for reducing the control gain only in the vicinity of a stop of the operation of the controlled object.
Contrary to Patent Literature 1, Patent Literature 2 proposes a technology for changing a control gain while a controlled object is operating.
That is, Patent Literature 2 discloses a technology for providing a model calculating unit (a reference model unit) that receives a position command as an input and outputs a model torque and a model position ideal for servo control, generating, based on a signal obtained by multiplying a positional deviation between the model position and an actual position of a controlled object by a position proportional gain, a feedback torque command (deviation compensation torque) to reduce the positional deviation, and adding up the feedback torque command and the model torque to generate a torque command for a motor.
In Patent Literature 2, assuming control of a truck called stacker crane, control for changing the position proportional gain to be smaller when the moving speed of the truck, that is, the speed of a motor is large than when the moving speed is small is performed for the purpose of preventing the control from too sensitively reacting to a phenomenon that, although highly accurate control is requested for positioning accuracy during a stop, wheels slip on the road surface in a high-speed state. Specifically, there is disclosed a technology for setting the position proportional gain as a polynomial based on the detection speed, the model speed obtained by differentiating the model position, and the like, to continuously change the position proportional gain according to the level of the speed.